We propose to test the hypothesis that IL-23 mediates a destructive inflammatory response to C. difficile infection (CDI). We further hypothesize in Aim 1 that inflammatory dendritic cells (DCs) produce IL-23 in the colon, and (Aim 2) that IL-23 in turn causes inflammation that disrupts the gut epithelial barrier, and that (Aim 3) the targets of IL-23 action are IL-23R expressing innate lymphoid cells (ILCs) that produce IL-17, IL- 22, and/ or IFN?. Successful completion of these studies will delineate the mechanism and pathways of IL-23 mediated exacerbation of CDI and provide several potential sites for therapeutic intervention. Significance: C. difficile is the number one hospital-acquired infection in North America and despite antibiotic therapy has a mortality of 10-15%, with relapses in 10-35% of patients. Innovative aspects of the proposal include that it explores the potential for immune modulators in the treatment of C. difficile. The environment for the work includes a team of graduate students in the lab of the Principal Investigator who has contributed to infectious colitis research for over 25 years, and an external advisor, Dr. Brian Kelsall, who brings special expertise in dendritic cell biology. R21: The scope is appropriate for an R21 as the work is at the earliest conceptual stages of understanding of IL-23 in CDI, and proposes a high risk/high reward study of its mechanisms of action.